The present invention relates generally to printing mechanisms, such as inkjet printers or inkjet plotters. Printing mechanisms often include an inkjet printhead which is capable of forming an image on many different types of media. The inkjet printhead ejects droplets of colored ink through a plurality of orifices and onto a given media as the media is advanced through a printzone. The printzone is defined by the plane created by the printhead orifices and any scanning or reciprocating movement the printhead may have back-and-forth and perpendicular to the movement of the media. Conventional methods for expelling ink from the printhead orifices, or nozzles, include piezo-electric and thermal techniques which are well-known to those skilled in the art. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, the Hewlett-Packard Company.
In a thermal inkjet system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are individually addressable and energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. The inkjet printhead nozzles are typically aligned in one or more linear arrays substantially parallel to the motion of the print media as the media travels through the printzone. The length of the linear nozzle arrays defines the maximum height, or xe2x80x9cswathxe2x80x9d height of an imaged bar that would be printed in a single pass of the printhead across the media if all of the nozzles were fired simultaneously and continuously as the printhead was moved through the printzone above the media.
Typically, the print media is advanced under the inkjet printhead and held stationary while the printhead passes along the width of the media, firing its nozzles as determined by a controller to form a desired image on an individual swath, or pass. The print media is usually advanced between passes of the reciprocating inkjet printhead in order to avoid uncertainty in the placement of the fired ink droplets. If the entire printable data for a given swath is printed in one pass of the printhead, and the media is advanced a distance equal to the maximum swath height in-between printhead passes, then the printing mechanism may achieve its maximum throughput.
Often, however, it is desirable to print only a portion of the data for a given swath, utilizing a fraction of the available nozzles and advancing the media a distance smaller than the maximum swath height so that the same or a different fraction of nozzles may fill in the gaps in the desired printed image which were intentionally left on the first pass. This process of separating the printable data into multiple passes utilizing subsets of the available nozzles is referred to by those skilled in the art as xe2x80x9cshingling,xe2x80x9d xe2x80x9cmasking,xe2x80x9d or using xe2x80x9cprint masks.xe2x80x9d While the use of print masks does lower the throughput of a printing system, it can provide offsetting benefits when image quality needs to be balanced against speed. For example, the use of print masks allows large solid color areas to be filled in gradually, on multiple passes, allowing the ink to dry in parts and avoiding the large-area soaking and resulting ripples, or xe2x80x9ccockle,xe2x80x9d in the print media that a single pass swath would cause.
A printing mechanism may have one or more inkjet printheads, corresponding to one or more colors, or xe2x80x9cprocess colorsxe2x80x9d as they are referred to in the art. For example, a typical inkjet printing system may have a single printhead with only black ink; or the system may have four printheads, one each with black, cyan, magenta, and yellow inks; or the system may have three printheads, one each with cyan, magenta, and yellow inks. Of course, there are many more combinations and quantities of possible printheads in inkjet printing systems, including seven and eight ink/printhead systems.
Each process color ink is ejected onto the print media in such a way that the drop size, relative position of the ink drops, and color of a small, discreet number of process inks are integrated by the naturally occurring visual response of the human eye to produce the effect of a large colors pace with millions of discernible colors and the effect of a nearly continuous tone. In fact, when these imaging techniques are performed properly by those skilled in the art, near-photographic quality images can be obtained on a variety of print media using only three to eight colors of ink. This high level of image quality depends on many factors, several of which include: consistent and small ink drop size, consistent ink drop trajectory from the printhead nozzle to the print media, and extremely reliable inkjet printhead nozzles which do not clog.
Unfortunately, however, there are many factors at work within the typical inkjet printing mechanism which may clog the inkjet nozzles, and inkjet nozzle failures may occur. For example, paper dust may collect on the nozzles and eventually clog them. Ink residue from ink aerosol or partially clogged nozzles may be spread by service station printhead scrapers into open nozzles, causing them to be clogged. Accumulated precipitates from the ink inside of the printhead may also occlude the ink channels and the nozzles. Additionally, the heater elements in a thermal inkjet printhead may fail to energize, despite the lack of an associated clogged nozzle, thereby causing the nozzle to fail.
Clogged or failed printhead nozzles result in objectionable and easily noticeable print quality defects such as banding (visible bands of different hues or colors in what would otherwise be a uniformly colored area) or voids in the image. In fact, inkjet printing systems are so sensitive to clogged nozzles, that a single clogged nozzle out of hundreds of nozzles is often noticeable and objectionable in the printed output.
It is possible, however, for an inkjet printing system to compensate for a missing nozzle by removing it from the printing mask and replacing it with an unused nozzle or a used nozzle on a later, overlapping pass, provided the inkjet system has a way to tell when a particular nozzle is not functioning. In order to detect whether an inkjet printhead nozzle is firing, a printing mechanism may be equipped with a low cost ink drop detection system, such as the one described in U.S. Pat. No. 6,086,190 assigned to the present assignee, Hewlett-Packard Company. The nozzle plate of a printhead is inherently near ground potential due to the power supply connections on the printhead. A conductive target may be placed a few millimeters below the nozzle plate, and a biasing voltage may be applied to the target, forming an electric field between the nozzle plate and the target. Upon firing an ink drop, as the ink drop begins to exit the nozzle, a charge accumulates on the protruding tip of the drop, due to the influence of the nozzle-plate-to-target electric field. When drop breakoff occurs, the drop retains this charge. When the drop contacts the target, a small current, in relation to the charge on the drop, is induced from the target to ground. The periodic flow of current from drops striking the target may be converted to a signal voltage by an amplifier which is AC-coupled to the target, and then an analog-to-digital converter may digitize the output signal for processing to determine if a nozzle or group of nozzles are working properly.
In practical implementation, however, this drop detection system has some limitations. Successive drops of ink, drying on top of one another quickly form stalagmites of dried ink which may grow toward the printhead. Since it is preferable to have the electrostatic sensing element very close to the printhead for more accurate readings, these stalagmites may eventually interfere with or permanently damage the printhead, adversely affecting print quality. Therefore, it is desirable to have a low cost and efficient method and mechanism for ink drop detection which is less susceptible to waste ink residue build-up.